For DC-DC converters, fast transient response is critical for high speed contemporary devices. Telecommunications systems, networking systems, servers, and processor-based systems are all examples of applications that employ high-speed devices. Microprocessors and custom ASICs (application-specific integrated-units), for example, have load requirements that can quickly change from a more or less idle state to full load and back to the idle state, demanding rapid increase and decrease in current. In addition, fast dynamic voltage transition may be required to raise or lower the regulated voltage depending on the operating mode. If a DC-DC converter is unable to quickly respond to the step-up and step-down load event or dynamic voltage transitions, the output voltage of the converter may go outside the desired regulation window, possibly causing damage or improper operation of the microprocessor or ASIC.
Some DC-DC converters have an active transient response (ATR) unit for mitigating step-down load events. These controllers are not limited by synchronous control of the power switches, where the pulse width duty cycle is modulated on a cycle-by-cycle basis to control the output voltage. In conventional buck converter synchronous control, the high side power transistor is in a conductive state (on) and the low side power transistor is in a non-conductive state (off) for a portion of a switch cycle, then the high side power transistor is off and the low side power transistor is on for the remainder of the switch cycle. Controllers with ATR units typically support both an ATR low (ATRL) state or an ATR high impedance (ATRL_HiZ) state to control voltage overshoot during a step-down load event.
In the ATRL state, the high-side power transistor is off and the low-side power transistor is on for one or more switch cycles, so that negative or positive current can flow from the output inductor through the low-side transistor in order to rapidly lower the output voltage. In the ATRL_HiZ state, both the high-side and low-side power transistors are off, so the converter power stage is switched to a high impedance (tri-state or HiZ) state so that the current in the output inductor more rapidly dissipates through the body diode of the low-side transistor, but the current cannot go negative in this state. Conventional ATR units employ or select one of these two states and do not switch from the ATRL_HiZ state to the ATRL low state when the inductor current reaches zero. However doing so can leave the ATR unit in the ATRL_HiZ state too long, which is problematic because it may lead to poor load transient or dynamic voltage response. It can also be problematic if drivers enter a low power sleep mode when its input is tri-stated for too long.